Aircraft landing system



Jan. 2, 1951 R. H. vARlAN ETAL 2,536,112

AIRCRAFT LANDING SYSTEM Original Filed Nov. 22, 1945 2 Sheets-Sheet 1 NToRs H. VAR/,4N TNO/14145 M. F E' R17/LL V/(KTTRNEY INVE ,1. if, A

.N .mwN R Jan. 2, 1

Original Filed Noy. 2.2, 1943 R. H. VARIAN ET AL AIRCRAFT LANDING SYSTEM 2 Sheets-Sheet 2 BY /M MATORNEY.

Patented Jan. 2, 1951 AIRCRAFT LANDING SYSTEM Russell H. Varian, Garden City, and 'llhomas `M. Ferrill, Jr., Hempstead, N. Y., assignors to The Sperry Corporation, a corporation of Delaware Continuation of application Serial No. 511,286, November 22, 1943. This application April 18, '1946, Serial No. 663,060

17 Claims.

This invention relatesl `generally to landing systems for aircraft in which a landing path is formed by a plurality of radio beams.

The present application is a continuation of patent application Serial No. 511,286 filed November 22, 1943, now abandoned.

In landing systems heretofore used, a landing ,path for Vthe aircraft has usually been formed by the intersection of a vertical plane and a plane inclined at an angle corresponding to the desired angle of the landing path. These vertical and inclined planes, sometimes referred to as localizer and glide path planes, may be produced inseveral different ways by radio beams radiating from localizer and glide path transmitters.

In a landing system of this type, the angle of inclination of the landing path is determined by the inclination of the glide path plane. Therefore, the glide path transmitter must be located in the vicinity of the intersection of the landing path with the ground which corresponds lapproximately to the ,point at which the plane lands.

Although the glide path transmitter is customarily located at one side of the contact point to avoid interference with aircraft which may be landing, it forms an obstruction lwhich may interfere with landings of other aircraft `on adjacent runways within a particular landing area. Since the glide path transmitter is arranged in the vicinity Aof the landing point, some difficulties have been encountered due to course sharpening as an 'approaching aircraft nears the landing point.

A major object of the present invention is to provide a landing system in which the transmitters dening a landing path may be Ylocated outside of the landing area.

Another object vof the invention is to provide a landing system in which the transmitters defining a landing path may be located at a point remote from the landing point.

A further object of the invention is to produce a landing path ina landing system by the intersection of two inclined signal surfaces.

A still ,further Aobject of the invention is to` provide an indicating system which converts the displacements relative to one pair of coordinate axes to corresponding displacements relative to a second pair of coordinate axes rotated by approximately 45 with respect to the first pair of axes.

Y The invention in another f its aspects relates to novel `features of the instrumentalities described herein for achieving the principal objects of the invention and to novel principles employed in those instrumentalities, whether or not these features and principles are used lfor the said principal objects or in the said eld.

A further object of `the invention is to provide improved apparatus and instrumentalities embodying novel 'features and principles, adapted for use in realizing the above objects and also adapted for use in other elds.

Other objects and advantages of the invention will become apparent from the following specification, taken in connection with the accompanying drawings, wherein Fig. 1 is va .perspective diagrammatic view of a landing area showing a landing path dened by radio beams, according to a .preferred form of the invention;

Fig. 2 is an elevation view of the landing area shown in Fig. 1 as seen from a point in front of the radiators;

Fig. 3 is a view of the radiators and their respective beams, as seen from the direction of the arrow in Fig. 1;

Fig. 4 is a schematic block diagram of an arrangement of a transmitter system suitable for producing a landing path according to the inventon;

Fig. 5 is a schematic circuit diagram of a receiver system adapted to be carried by an aircraft for use in directing the aircraft along the landing path shown in Fig. l;

Fig. 6 is a schematic circuit diagram of a modified receiver system having a special circuit for the received signals to actuate a normally oriented cross pointer indicator;

Fig. '7 is a perspective view of a single-beam directive radiator;

Fig. 8 is a perspective view of a dual-beam radiator; and

Fig. 9 is an elevation of a pair of single-beam directive radiators oriented for `producing an inclined equisignal surface.

According to the invention in its preferred form, directional radiators are positioned beyond one end of a runway or landing strip to produce two signal surfaces, such as equisignal planes, arranged symmetrically relative to the landing runway and intersecting to form a landing path. The radiators are preferably positioned so the equisignal planes are inclined at equal angles relative to the landing area and so directed relative lto each other that the landing path intersects the runway at the most desirable landing point along the length of the runway.

In this manner the landing path is formed by the intersection of two equisignal planes or signal surfaces inclined to a horizontal plane or the plane of the landing area. Thus, the signal surfaces whose intersection defines the landing path are both angularly disposed with respect to a lateral plane including the landing path, that is a plane including the landing path and a horizontal line perpendicular thereto. The inclination of the landing path depends upon the angle of linclination of both planes and the horizontal angle between them. As shown in Fig. 1, radio beams are arranged to form a landing path for a landing strip Il which may be one of many landing strips within a Vparticular landing area. Four radiators of electromagnetic energy l2, I3, I4 and l5 are arranged in two pairs, one pair on each side of the landing strip and beyond the end of the strip. In the drawing the end of the landing strip is considered the end of the landing area, so the radiators are positioned just beyond the landing area. As Will become apparent, the radiators may be placed at a much greater distance from the landing strip if desired.

The radiators may be of the type shown in Fig. '7, in Which electromagnetic energy is supplied from a suitable transmitter as by a wave guide I6 which emits high frequency energy at the focal point of a reflector il having the form of a cylindrical parabola. As is well known, a reflector of this type provides a fan-shaped beam of energy having a substantially elliptical crosssectional intensity contour, i. e. a relatively thin pattern of appreciable width, as exemplied in the dotted line cross-sectional outlines in Fig'. 2;v

and the similar outline representations at the left-hand end of Fig. l.

The two radiators I2 and I3 on one side of the landing strip I I are arranged one above the other and have their directivity axes 22 and 2i (Fig. 3) angularly offset to project beams i8 and I9 of radiant energy which produce a signal surface in the form of an equisignal plane 22. This equisignal plane 22' is the locus of points at which the energy of both of the beams is at the same intensity level.

As shown in Fig. 2, the radiators i2 and I3 are preferably inclined so the equisignal plane 22 produced by means of i8 and I9 is inclined to the plane of `the landing area at an angle of approximately 45 degrees. The radiators I2 and i3 are directed, by turning them together about a vertical axis, so the equisignal plane 22 intersects a landing point 2S which corresponds approximately to the point at which it is desired for an approaching aircraft to land. The radiators I2 and I3 may then be adjusted about an axis perpendicular to the equisignal plane 22 to direct the maximum strength of the radiators as desired to provide the best reception by an approaching aircraft.

The radiators I2 and l5 are arranged on the opposite side of the landing strip to project beams 213 and 25 of electromagnetic energy Which have their axes offset so the beams form a signal surface such as equisignal plane 22 corresponding to the equisignal plane 2'2 formed by the beams 26, 2i. The radiators I4 and I5 are inclined to the landing area so the plane 26 is arranged at any desired angle, preferably 45 degrees. The radiators I2 and I5 are directed, by turning them together about a vertical axis, so the equisignal plane 25 passes through the landing point 23. When thus arranged, the equisignal plane 26 intersects the equisignal plane 22 to form a landing path 2l.

This landing path, since it is formed by the intersection of two planes which pass through the landing point 23, must also pass through the landing point 23. It will be apparent, therefore, that the intersection of equisignal planes or signal surfaces 22 and 22 forms a landing path 2'? for guiding an approaching aircraft 28 to land at the point 23 on the landing strip i i One arrangement of transmitters for supplying energy to the radiators is shown in the schematic block diagram of Fig. 4. A transmitter 3l generates high frequency energy of a carrier frequency F1 which is connected to a pair of modulators 32 and 33 that supply the high frequency carrier modulated by signal frequencies f1 and f2, preferably audio frequencies, to the radiators I2 and I3, respectively. The radiators I2 and I3 thus radiate energy of the same carrier frequency F1 in two beams I8 and I9 which are modulated at audio signal frequencies f1 and f2, respectively.

In a similar manner a transmitter 35 supplies high-frequency energy of a frequency F2 to modulators 3'6 and 3l, which modulate the highfrequency energy generated by the transmitter 35 at audio signal frequencies f3 and f4, respectively.

The energy generated by the transmitters 3i and 35 is preferably of slightly different frequencies to avoid interference between the beams radiated by the two pairs of antennas. However, this is not essential to the present invention. It is only necessary that receivers carried by the aircraft be able to identify each of the four beams either by differences in the carrier frequencies or by differences in the modulating signals.

Since the transmitters are shown Yas having different carrier frequencies F1 and F2, receivers 38 and 39 (Fig. 5) connected to antennas 4l and 42, that are carried by aircraft using the landing system, are tuned to receive carrier frequencies F1 and F2, respectively. The receiver 38 is connected to a pair of band-pass filters G3 and 44 which are adapted to pass signal frequencies f1 and f2, respectively. The outputs of these filters 23 and i2 are connected across rectifier bridge circuits 15 and 46 which have their outputs connected in opposition, through resistors #il and 28 to one-pointer 49 of a cross-pointer indicator 5I. The voltages produced by the rectifier circuits 25 and 46 depend upon the intensity of the energy modulated by frequencies f1 and f2 as received from beams I8 and i9, respectively. Therefore, the voltages of these rectiers, which are applied in opposition to the pointer1 39, depend upon the position of the aircraft relative to the equisignal plane 22.

Thus, the polarity and magnitude of the voltage applied to the pointer depends upon the direction and magnitude of the displacement of the aircraft relative to the equisignal plane 22. It will be apparent that the pointer I9 is displaced from its neutral position in a direction and an amount corresponding to the direction and amount of the displacement of the aircraft from the equisignal plane 22.

In a similar manner the receiver 39, which is tuned to the carrier frequency F2, is connected to band-pass lters 53 and 54 that respond to signal frequencies f3 and ,f4 and have their outputs supplied-to bridge rectifier circuits 55 and 52, respectively. The outputs of the bridge rectier circuits 55 and 5B, that depend upon the intensity of energy received from beams 2li and 25, are connected in opposition through resistors 51 and 58 to the other pointer 5S of the crosspointer indicator 5i. In this manner, the pointer 5S is displaced from its neutral position in a direction and an amount corresponding to the direction and amount of displacement of the aircraft-relative to the equisignal plane 26.

The cross-pointer indicator 5I is mounted in such a way that its pointers 49 and 59 are inclined to the horizontal in substantially the same manner as the equisignal planes 22 and 26 to which they respond. In this way the intersection of the pointers relative to a reference mark 60 indicates the displacement between the aircraft and the landing path.

This arrangement of the pointers of a crosspointer indicator differs somewhat from the system conventionally employed in instrument landing systems. Since the displacement of the aircrat `from .either of the signal surfaces includes both horizontal and vertical components, deviation of either pointer .from its neutral posi- -tion Vcannot easily Ibe corrected by .a single adjustment of only one of the Yconventional controls :for `the aircraft. This .may be advantageons under .certain conditions, .in that .it requires the pilot to :devote equal attention to both winters.

.order -that pilots trained to use a crosspointer indicator having .horizontal and vertical pointers may use the landing system, 'a receiver such as that shown in Fig. `6 may vbe employed. This :receiver .determines the displacement ,of the aircraft relative to the inclined equisignalrplanes 22 and 26 .in 'the same manner as the receiver shown in Fig. 5, but converts the displacements thus determined into vertical and lateral displacements of the aircraft relative to the landing path. These vertical and lateral displacements operate horizontal and vertical pointers of 4a .cross-pointer indicator mounted in a `conventional manner in the aircraft.

The receiving apparatus shown .in Fig. 6 .includes :receivers .6| and 62, connected to antennas 63 and v(ill and tuned to Vcarrier frequencies F1 and Farespectively. for detecting enere'yreceived from each pair of beams. The receiver 6| detects :energy .from the radiators l2 and I3 and supplies it to `band-pass lters A65 and 66, which pass energy modulated by frequencies f1 and f2, respectively. The band-pass filters BE and 5S are connected across bridge detector circuits 61 and 68 Whose outputs are connected in .opposition through `resistors .69 and 'H to a double bridge circuit `designated generally7 at 1:2.

.In a similar manner the receiver 62 detects energy having :a carrier frequency .F2 radiated by vradiators M and I5 and supplies these detection signals to band-pass filters 13 and 14 which pass signals of frequencies f3 and f4, respectively. The outputs of band-pass filters 13 and 14 are connected across bridge rectifying circuits -15 and 1E, the ioutputs of which are connected in opposition `.through resistors 1:1 and 1S to the double bridge circuit 12.

.As previously explained in connection with Fig. f5, the `voltage produced by the combination of the outputs of rectifier circuits in opposition corresponds in polarity and magnitude to the direction and amount of displacement of the aircraft relative to the respective equisignal planes which define the landing path. This displacementis, of course, measured as the shortest distance from each plane. In order to convert the displacement lof the aircraft relative to the inclined equisignal planes 22 and 25, as represented by the combined output of the rectifier circuits 61 Vand 68 and the combined output of the rectier Acircuits 15 and 1G, respectively, Athe sum of these outputs is used to actuate one pointer of the cross-pointer indicator 8l and the diierence of these outputs is used to actuate the other pointer of Vthe indicator 8|. The combination of the outputs in sum and .dinerence circuits is accomplished by the double bridge circuits 12.

From an examination of Vthe circuit A1,2, as shownin Fig. f6, 'a combined output of .the Arectier lcircuits'tiT and E8, representing the displacement of the .aircraft relative `to the equisignal plane 22 is supplied through resistors 33 and 8d and leads 85 and 85 to the vertical pointer `Si! of the cross-pointer indicator 8|. The combined outputs of therectier .circuits 15 and 1B, :repreist 6 sent-ing the displacement of `the aircraft 'from the equisignal planel is added to that .from the :other rectier circuits through resistors 89 and to the leads f8.5 and 8S which are connected to :the vertical pointer 81.

The combined output of rectifiers 61 and 63 is also connected through .resistors 92 and 93 and leads i913 and 95 to horizontal pointer of the indicator 8|.. Opposed to these signals is the .combined 'output :of rectiiiers 15 and 1S which `is connected .in opposition through resistors .91 and .9.8 to the leads .9.5 and :94 which actuate the `horizontal pointer 96.

Although .the ,circuit .12 shows Ythe sum of the outputs of 'rectier `circuits appliedfto the vertical pointer 81 and the difference of these outputs 'applied to the horizontal pointer 95, it will be obvious that the circuit could ce reversed to apply the difference to the vertical pointer and the sum to the horizontal pointer without departing from the invention.

The receiving apparatus shown `in Fig. 6 determines the displacement `of the 'aircraft relative tothe inclined equisignal planes 22 `and 26 a. d converts 'these displacements to lateral and vertical displacements of the aircraft from the ianding path 21. IThese -lateral and vertical displacements actuate normally vertical `and normally horizontal pointers 81 and B of the crosspointer indicator fl. This system provides a pilot with the same indication for 'landing his aircraft as that provided ybya cross-pointer meter which has its vertical .and 'horizontal pointers controlled by energy .received from localizer and glide path transmitters. Gbviously, this convertsystem may .also beused'with a glide path `and localiser landing system 'to actuate a crosspointer indicator having inclined pointers. In this case,-the sum and diflerenceof the glide path and lccalizer signals are converted to actuate the inclined pointers of vthe indicator.

The displacements of .the craft from rthe equisignal planes may be 4considered as displacements relative to a pair of coordinate axes, which correspond to the lines formed by the intersection of the equisignal planes including the position of the craft and perpendicular `to 'the equisignal planes. `@ne indicating pointer is actuated according A:to -the sum .and another indicating pointer is actuated according to the dinerence of the displacements relative to the pair of coordinate axes to indicate the position of the aircraft relative to a second pair of coordinate axes, which, in the present case, are horizontal and vertical axes.

By the arrangement described, the indicating system corrects displacements of an object relative to one pair of coordinate axes to displacements relative to a second pair of 'ai-:es that are angularly displaced with respect to the rst pair. In the preferred form, the coordinate axes in both cases are mutually perpendicular and the two pairs of axes are angularly displaced by forty-five degrees. This is due to the fact that the 'indicators indicate the displacements of the object relative to a second pair of axes which bisect the angles formed by the rst pair of axes. Hence, if the rst coordinate axes are not mutually perpendicular, the second axes are not rotated by forty-five degrees. The angle of Vrotation depends upon `the angles formed by the first pair of axes, since the second `pair bisects these angles.

The equisignal planes 22 and 25 may .be inclined .at .any desired angle to the .landing area.

As previously stated, these planes are preferably arranged symmetrically relative to the landing strip li. When the equisignal planes 22 and 26 are arranged at forty-ve degree angles, as shown in the drawings, a substantially uniform sensitivity is provided at the receiver for displacement of the aircraft in all directions from the landing path 2l. If both of the equisignal planes 22 and 25 are inclined at an angle other than forty-five degrees, the ratio of Vertical to lateral sensitivity is changed. This may be desirable in particular circumstances. However, for more universal use, the arrangement of the radiators to produce equisignal planes inclined at angles of forty-ve degrees is preferred.

Although the equisignal planes are preferably arranged symmetrically relative to the landing strip, it will be apparent to those skilled in the Vart that the landing path may be defined by the intersection of planes inclined at different angles without departing from the invention.

It is contemplated that many different types of radiators may be substituted for those shown in the accompanying drawings. For example, it is not essential that two reflectors be used on each side of the landing strip Il. As is now well known, and as shown in Fig. 8, a single reflector Il of the same type as reflector Il shown in Fig. 7 may be supplied by two wave guides 80 and 82 which radiate high frequency energy at points slightly offset relative to the focal point of the reflector. This single reflector type of radiator projects a pair of divergent beams of radio energy in substantially the same manner as the dual radiator system already described. Energy emerging from upper wave guide 8d at a. first point in the focal region of reflector Il is reflected by the reflector Il' to form a lower directive beam and energy emerging from wave guide 32 at a point slightly lower than said first point is reflected by the reflector il to form an upper directive beam. If a dual-beam radiator such as that illustrated in Fig. 8 is employed instead of a pair of single-beam radiators l2 and i3 (Fig. 4), for example, wave guide 8l) is coupled tothe lower-beam modulator 32 and wave guide 82 is coupled to the upper-beam modulator 33. Another dual-beam radiator of the same type may be employed instead of the single-beam radiators lli and'l associated with transmitter 35 and modulators 3e and 31, Fig. 4.

Where two separate single-beam radiators are employed for defining each equisignal plane or surface, the relative orientations of the radiators may be readily fixed according to the desired orientation of the equisignal surface. To accomplish this, the reflectors may be arranged vertically, horizontally, or at any angle, with their directivity axes substantially parallel. An equisignal plane may then be provided at any desired inclination to the landing area by turning the radiators relative to each other about an axis in the desired equisignal plane. As illustrated in Fig. 9, for example, two directive radiators l2' and I3', each similar to the radiator it, Il shown in Fig. 7, are arranged in parallel relation at an angle of inclination of approximately 45 degrees. The directive axes of these radiators originally were aimed along substantially parallel lines. Radiator l2 was thereafter rotated through a very small angle in a clockwise direction, and radiator i3 was rotated through an equal angle in a counterclockwise direction. These radiators are positioned to generate a pair of divergent directivity patterns or beams dening an inclined equisignal plane or surface which contains the par# allel lines along which the radiators l2 and I3.' originally were aimed.

Although the system heretofore described defines the landing path by the intersection of inclined signal surfaces in the form of two equisignal planes, it is contemplated that the invention maybe applied to a system employing curved equisignal surfaces, as distinguished from planar surfaces. For example, equisignal surfaces may be developed by concentric beams of circular or elliptical cross-section. Thus, if two beams, such as those shown in Fig. 1 are projected so their axes are coincident, but one beam is slightly broader than the other, an equisignal surface having a substantially elliptically curved crosssection is produced `between the two beams.

Obviously, the intersection of such a curved equisignal surface could be used in a system embodying the present invention. Curved equisignal surfaces used in this manner have their sur# faces inclined in a direction normal to the line of intersection, at a greater angle relative to a horizontal plane through a particular point along the intersection than the angle of inclination of the landing path defined by the intersection. That is, the curved surfaces are inclined at the line of intersection so tangents to the surfaces are angularly inclined with respect to a lateral plane through the landing path. Equisignal surfaces may thus be produced by transmitter arrangements in a manner quite similar to that shown in Fig. l with the radiators positioned on each side and beyond the end of the runway to clear the landing area of all obstructions.

Any constant intensity level of a single beam, such as one of those shown in the drawings, forms a signal surface the shape of which depends upon the characteristic of the radiator. By project-4 ing one beam from each side of the runway, the intersection of these constant intensity signal surfaces also forms a landing path, Here again it is necessary to have the angle of inclination of the signal surfaces alo-ng their line of intersection greater than the inclination of the landing path at a given point. In this manner, the angle of inclination of the signal surfaces, as measured by the angle between a line contained within said surfacesand normal to the` landing path and a horizontal plane, is greater than the inclination of the landing path relative to the horizontal plane.

In order to fly an aircraft along a landing path defined by the intersection of two constant intensity signal surfaces, it is necessary to modify the receivers shown in Figs. 5 and 6. However, receivers for constant intensity work are of con ventional design arranged to deflect the pointers of a cross-pointer meter to their1 central positions for a selected intensity of the signals received from a particular beam. The pointers of a crosspointer indicator may thus be deiiected more or less depending upon whether the intensity is greater or smaller than that of the signal surfaces of the intersection forming the landing path.l

A converter similar to that shown in the receiver apparatus of Fig. 6 may also be employed in a constant intensity landingsystem embodying the invention for transforming displacements from one pair of coordinate axes to vactuate vertical and horizontal pointers of a cross-A pointer indicator.

From the foregoing description, it will be apparent that the present invention permits the arrangement of radiators remotely from the landing point, that is, the point kat which .the 'landing path intercepts the runway. This is :ac- `complished Eby forming a landing path by the intersectioniof two inclined signal surfaces of .electromagnetic energy. 'These signal surfaces are preferably incl-ined at 'equal angles `with respect to alateral plane through the landing path, that is, they are incl-inedsymmetrically reiative to the direction Yof gravity. However, they may `be in.- v4clined yat any Aangle to the lateral `plane without Ideparting from the invention. The receiverscarriedby the aircraft to determine its displacement `relative to the landing :path `may `employ nondirectional receiving antennas so the cross-pointer indicator is not aected bychanges in-the attitude of the aircraft.

As many changes could be made vin the yabove lconstruction and vmany :apparently widely different Aembodiments of this invention could y.be made without departing fre-m the scope thereof, it -is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. In a landing system for an aircraft,-appara tus for defining a landing path comprising means for producing an equisignal surface of electromagnetic energy, and means for producing another equisignal surface of electromagnetic -energy intersecting the first `surface to form a landing path, said signal surfaces -being inclined at a point on said path with respect to a horizontal plane passing through said point at an angle greater than the inclination of the landing path at said point with respect to said horizontal plane.

2. In a landing system Vfor an aircraft, apparatus for defining a `landing path comprising means for projecting a pair of beams of electromagnetic energy `to form an equisignal surface `between said beams, Aand means for projecting another pair of beams of electromagnetic Aenergy to'form another equisignal surface between said beams intersecting said first signal surface to form a landing path, said beams being arranged so said equisignal surfaces are inclinedfat `said intersection with respect to va `horizontal plane at 'angles greater than the inclination of said landing path.

3. -In a landing system for an aircraft,appara tusfor defining a landing path comprising'means forproducing an equisignal plane-of electromagnetic energy, and means for producing another equisignal plane of electromagnetic energy intel'- secting said first equisignal plane to form a landing path, said equisignal planes being inclined with respect to a horizontal plane at angles greater than the inclination of said landing path.

4. The method of forming a landing path for an aircraft which comprises projecting a pair of beams of electromagnetic energy to provide an equisignal surface between said beams, and projecting another pair of beams of electromagnetic energy to provide another `eeluisignal surface b etween said beams intersecting said iirst equisignal surface to form alanding path, said beams being arranged in a manner such that said equisignal surfaces are inclined atsaid intersection with respect to a horizontal plane at angles greater than the inclination of said landing path.

5. The .method of forming a landing path for an aircraft which comprises producing an equisignal plane of electromagnetic energy, and producing another equisignal plane of electromagnetic energy intersecting said first equisignal plane to form a landing path, said equisignal planes being inclined with respect to a horizontal :plane at angles greater than the inclination -of said landing path.

6. In a landing system for an aircraft, apparatus for `indicating the position of said aircraft relative to a landing path defined by two surfaces of electromagnetic energy comprising a cross-pointer indicator, and means for actuating `one pointer of said 'indicator according to the sum, 'and the Aother pointer of said indicator according to the difference -of the displacements of said aircraft ,relative to said surfaces.

7. In a landing system for .an aircraft, apparatus for `indicating the position of said aircraft relative to a landing path defined by two surfaces of electromagnetic energy comprising a receiver for producing signals corresponding to the dis- `placement of said aircraft relative to said surfaces, a cross-pointer indicator and means con- -necting said cross-pointer indicator to said receiver for moving one pointer according to the sum of said signals and the other pointer according to the difference of said signals.

8. In `a. landing system for an aircraft having beams of electromagnetic energy providing two inclined surf-aces intersecting to-deiine a landing path, a cross-pointer indicator having vvertical and horizontal pointers for `indicating Yvertical and horizontal displacement of said aircraft relative `to said landing path, means -for actuating vone -of said pointers according to the Vsum of the displacements Vof said aircraft relative to said surfaces, -and means for actuating the lother of said pointers according to the difference of the displacements of said `craft relative to said surfaces.

9. In a landing system for an aircraft 4having two pairs of beams, defining a pair of equisignal surfaces intersecting lto form a landing path, apparatus for indicating the position of said -aircraft relative to said Alanding path, comprising means actuated by a receiver Vaccording `to lthe strength of signals from one Vpair of said beams for `producing signals corresponding to the 'displacement of said craftfrom oneof `saidfequisignal planes, means actuated `by a receiver according to the strength fof signais yfrom the second pair of beams for producingsignals corresponding to the displacement of said aircraft relative to the second of said equisignal planes, a crosspointer indicator, a 4circuit connecting said means for actuating one pointer of said Ycrosspointer indicator according to the sum of said signals, and a circuit for connecting said means for actuating the other pointer of said crosspointer indicator according to the difference of said signals.

10. In an indicating system, apparatus for transforming displacements measured relative to one pair of coordinate Yaxes into corresponding displacements measured relative -to a second pair of coordinate axes bisecting the-angles formed by said one pair of coordinate axes, comprising a pair `of devices `for indicating displacements relative to lsaid second pair of axes, and means for energizing one of said devices according to the sum of the displacements relative to said first pair of axes and the other of said Vdevices according to the difference of the displacements relative'to said second pair of axes.

Algl. The method of indicating the position of an aircraft relative toa landing path defined by two surfaces of electromagnetic energy which comprise positioning one pointer of a cross-pointer in- '11 dicator according to the sum, and the other pointer of said indicator according to the difference of the displacements of said aircraft relative to said surfaces.

12. The method of indicating the position of an object which comprises the steps of producing signals corresponding to the displacements of the object relative to one pair of coordinate axes and utilizing said signals to respectively actuate a pair of indicating devices according to the sum and dierence of said displacements for indicating the position of the object relative to a second pair of coordinate axes bisecting the angles formed by said first pair of axes.

13. In a landing system for aircraft, apparatus for defining a landing path comprising first directive transmitting means for producing a first signal energy distribution in space, second directive transmitting means for producing a second signal energy distribution in space, the energy of said second distribution being distinctive from the energy of said first distribution, said first and second directive transmitting means being symmetrical about a vertical plane wherein the landing path is to be defined, said first and second directive transmitting means being aimed along a direction in said vertical plane of symmetry inclined upward at a very shallow angle of elevation, and said first and second energy distributions extending generally along said inclined direction from said first and second directive transmitting means and defining a guidance path along said inclined direction, said first directive transmitting means comprising means for dening a first relatively thin signal energy surface of appreciable width inclined steeply transverse said Vertical plane and said second directive transmitting means comprising means for defining a second relatively thin signal energy surface of appreciable width inclined steeply transverse said vertical plane and intersecting said first signal energy surface in said plane and there defining said landing path.

14. In a landing system for aircraft, apparatus for defining a landing path comprising first directive transmitting means for producing a first signal energy distribution in space, second directive transmitting means for producing a second signal energy distribution in space, the energy of said second distribution being distinguished from the energy of said first distribution, said first and second directive transmitting means being symmetrical about a vertical plane wherein the landing path is to be defined and comprising means for directing the energy of said first and second distributions in fan shaped beams aimed substantially in the direction from which the aircraft is to be guided downward along the landing path, said fan shaped beams being steeply inclined in the section taken in the vertical plane .transverse said plane of symmetry.

15. In a landing system for aircraft, apparatus according to claim 14 for defining a landing path, said first and second directive transmitting means each comprising means producing a second fan shaped beam also directed generally in the direction from which the aircraft is to be guided downward and forming an equi-signal surface with the respective ones of the beams defined in claim 14.

16. In a landing system for aircraft, apparatus for defining a landing path comprising first directive transmitting means for producing a first signal energy distribution in space, second directivel l2 transmitting means for producing a second signal energy distribution in space, the energy of said second distribution being distinguished from the energy of said first distribution, said first and second directive transmitting means being symmetrical about a vertical plane wherein the landing path is to be defined and comprising means for concentrating the energy of said first and second distributions toward a distant point on the landing path from which the aircraft is to approach therealong, the region of maximum intensity of the energy of said first distribution extending in a substantially fan shaped locus having a lower boundary extending along the ground through the landing point and the region of maximum intensity of the second distribution also being in a fan shaped locus having a lower boundary extending along the ground through the landing point and there intersecting the locus of the first beam.

17. In a landing system for aircraft, apparatus for defining a landing path comprising first directive transmitting means for producing a rst signal energy distribution in space, second directive transmitting means for producing a second signal energy distribution in space, the energy of said second distribution being distinctive from the energy of said first distribution, said first and second directive transmitting means being symmetrical about a vertical plane wherein the landing path is to be defined, said first and second directive transmitting means being aimed along a direction in said vertical plane of symmetry inclined upward at a very shallow angle of elevation, and said first and Second energy distributions extending generally along said inclined direction from said first and second directive transmitting means and defining a guidance path along said inclined direction, said first directive transmitting means defining a first signal energy surface inclined steeply transverse said vertical plane and said second directive transmitting means comprising means for defining a second signal energy surface inclined steeply transverse said REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,872,975 Kolster Aug. 23, 1932 2,097,072 Lock Oct. 26, 1937 2,165,256 Hansell July ll, 1939 2,179,570 Zublin Nov. 14, 1939 2,241,907 Hahnemann et al. May 13, 1941 2,242,910 Hahnemann May 20, 1941 2,294,882 Alford Sept. 8, 1942 2,435,988 Varian Feb. 17, 1948 

